The reaction of amines with phosgene to produce isocyanates is well known. The reaction may be represented by the following general reaction: ##STR1## In the course of the reaction the intermediate carbamyl chloride is formed which has a tendency to react under normal reaction conditions to produce urea and tars which detract substantially from the yield of the desired isocyanate. To avoid the formation of these side products several improvements in the phosgene preparation of isocyanates have been proposed.
One prior art method calls for a two-stage process, the first stage entails the formation of a slurry of intermediates at temperatures ranging from 0.degree. C. to room temperature and subsequently reacting the intermediate products with phosgene at temperatures high enough to convert the intermediate to the isocyanate, usually in the range of 160.degree. to 200.degree. C. This procedure presents processing difficulties due to the release of large amounts of phosgene when the temperature is elevated in the course of the reaction.
Another prior art method is that of U.S. Pat. No. 2,908,703 wherein efforts to minimize by-product formation are by means of a two-stage procedure involving a first stage reaction at a temperature of from about 60.degree. C. to about 90.degree. C. and a second stage wherein intermediate product from the first stage are further reacted.
Still another method attempted is that of U.S. Pat. No. 3,226,410 wherein the patentees describe a continuous process for producing diisocyanates aimed at minimizing backmixing by reacting the phosgene with a dilute stream of the amine carried in an inert organic diluent under superatmospheric pressure in a controlled turbulent flow. None of the known prior art methods have sufficiently reduced the undesirable by-product formation. There is thus a need for a suitable method to increase the yield by minimizing by-products in the manufacture of diisocyanates.